KUNUL. SV. VET. AKADEMIEXS IIAXDL. BAND. 19. X:0 6. 31 



as yet has been ascertained. A few tubes from the shale beds of Wisby and Oster- 

 garn might possibly once have appertained to that group, but their imperfect condi- 

 tion precludes identification. 



As genera especially characteristic of the Silurian beds of Gotland, may be men- 

 tioned: Craspedostoma, Onychochilus and probably also Pycnoinphalus, though it is 

 not certain whether the last does not occur in England and Oesel, but referred to the 

 genus Platyschisma in a conception which is not that of the tirst author. Autodetus, 

 which is spread over the whole island, occurs also on the neighbouring Oesel, though it 

 is uncertain whether so common there as on Gotland and not recorded from any other 

 Silurian region except Scania. As characteristic of the Upper Silurian formation in 

 general may be annotated Chelodes, which has also been found in Bohemia, Tremano- 

 tus, Cyrtolites and Subulites also known from N. America and Canada, Subulites- also 

 from England, and Tryblidium found in Estland and Canada. 



Concerning the many palfeozoic genera, of Avhich I have appended an Index in 

 the end of this memoir, it depends much on individual opinions, as to what genus these 

 old fossil shells are to be referred, but it is evident that it is highly futile to establish 

 the genera only on a single and variable character. When comparing large numbers of spe- 

 cimens belonging to the same species and collected in the same locality the great variabi- 

 lity in the form of the aperture, the columella, the height of the spire, the umbilicus, will 

 be evident, and it is only necessary to remind of such forms as Platyceras cornutum 

 to discover the wide range within which some species vary, chiefl}- those which have 

 a lai'ge horizontal and vertical distribution. In almost the same degree Cycloneraa 

 delicatulum varies with high or low spire, slender or ventricose whorls, with open or 

 closed umbilicus. 



Among structural peculiarities ought to be observed that in the genus Euom- 

 phalus, as it has been long known, the shell, in the rule, near the apex or in the 

 oldest whorls, is divided into one or several compartments or chambers by concave 

 diaphragms, without, however, having an}' communication or sipho between them. The 

 chambered apex has very often through the fossilisation become deciduous and the 

 blunt end indicates the place, where the partition once traversed the shell. In Murchi- 

 sonia, Loxonema and a presumed Trochus, Tr. gotlandicus, the apex is filled with a 

 compact mass of calcareous matter quite as in the recent Magilus. 



The peculiar and cellular structure of the external walls in Autodetus is a feature 

 well desei'ving attention. In a few genera, as Platyceras, Pleurotomaria, Euomphalus, 

 there is a tendency in several species to form scalarid varieties and in some species 

 all specimens found have assumed that peculiarity in growth. In the large genus 

 Pleurotomaria it is easy to follow the morphological changes which the characteristic 

 slit band is subject to, from a concave groove to the large, thin lamina in the group 

 .\latEe. In respect to the sculpture of the surface I have called the spiral lines, keels 

 and ridges, which run from the apex to the aperture for longitudinal, as they foUoAv 

 the shell in its whole length, and, consequently, those intersecting them are to be 

 denominated transverse. Some authors call all the later for lines of growth. But 

 besides the real lines of growth or the old apertural lips, which are conspicuous by 



